Nature, source and composition of volcanic ash in surficial sediments around the Zhongsha Islands

Abstract

Volcanic detrital sediments are a unique indicator for reconstructing the petrogenetic evolution of submarine volcanic terrains. Volcanic ash in surficial sediments around the Zhongsha Islands includes three kinds of volcanogenic detritus, i.e., brown volcanic glass, colorless volcanic glass and volcanic scoria. The major element characteristics show that bimodal volcanic activity may have taken place in the northern margin of the South China Sea, with brown volcanic glass and colorless volcanic glass representing the mafic end-member and felsic end-member, respectively. Fractional crystallization is the main process for magma evolution. The nature of the volcanic activity implies that the origin of volcanic activity was related to extensional tectonic settings, which is corresponding to an extensional geodynamic setting in the Xisha Trench, and supports the notion, which is based on geophysical data and petrology, that there may exist a mantle plume around the Hainan Island.